Note: Descriptions are shown in the official language in which they were submitted.
P~IN 88l~6
AUDA/CB
30.12.1977
~4~67
`~ ~a-r-~n~ lm r~ r
The invention relates to a carbon
film resistor comprising a non-conducting substrate provid~d
with a carbon film, a top coating of inorganic m~terial
and an electrically insulating layer and electrodes which
contact the carbon film.
United States Patent Specification
3,244,559 discloses a carbon film resistor of this type.
With this carbon film resistor the electricallr insulating
layer which may, for example, consist of a glass envelope,
hermetically s0als the subjacent c~rbon film with top coa-
ting. With this prior art carbon film resistor the top
coating consists of a high-melting metal oxide obtained by
decomposing an organic compound. The envelope of glass is
; necessarr because the top coating o~ a high-melting oxide,
for example, silicon dioxide, af-ter the resistance has
been brought to the desired value by locally removing,
scoring etc. of~ the carbon film no longer protects the side
faces of the carbon f~m then exposed from attack by the
atmosphere.
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It is an object of the invention to
pro~ide qualitatlvely comparable resistors of a simpler
construction. A carbon film resistor which satisfies this
o4~ct is charaotsrlsed in that the top ooating consists of
silicon nitride and the envelope of ~n organic lacquer layer~
,
The reslstance can bs brought in the customary mannar to
the desirsd valus by grinding, scoring or otherw-se
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30.12.1977
467
locally removing the carbon film together with th-e top
coating. It wa-s surprisingly found that an envelope of
an or~anic lacquer-layer is sufficient, and that a hermetic
seal as required for the construction described in the
above-mentioned United States Patent Specification 3,24LI,
559 can be dispensed with.
The resistors are produced in the
customary manner;
Cylindrical bodies of a ceramic
material are provided with a carbon film by means of a
Chemical Vapour Deposition process~ Thereafter a silicon
nitride layer is applied also by means of a chemical
vapour deposition process. The thickness cf the deposited
- layer may, for example~ be between 0.05 and 0.5/um.
EIereafter cap-shaped electrodes are pushed on the ends of
the cylinder. The resistors are brought to the desired
~lue by removing a portion of the carbon film, whilst the
resistance value is continuously measuredc The envelope
is obtained by coating the resistors with one single
; 20 lacquer layer. The lacquer may, for example, consist of
an epoxy resin. The resistors according to the invention
were subjected to an accelera~ed i'moisture" test. It was
~ound that of resistors having a resistance value exceeding
1000 k ~ after having been immersed in boiling water for 1
hour the resistance value increases by less than 1 % if
~hereafter the resistor is charged to the rated vo1tage
.
If no silicon nitride layer is applied and only five super-
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PHN 8846
30.12.1977
4~7
imposed lacquer layers of a high quality are app~ed
then the resistance valu~ increases in these circumstances
by at least 10 %~ In some cases the carbon film is then
even interrupted.
The resistors according to the
invention are well protected from overloading and little
sensitive to electrochemical corosion. They have a long
life at an elevated opercting temperature.
l`he invention will be described in
greater detail ~ith ~ference to the accompanying drawings,
the sole Figure of which shows in cross-section an embodi-
ment of a carbon film resistor according to the invention.
In the Figure the numerals have the
following meaning.
1 = cylindrical ceramic body
2 = ¢arbnn film
3 = top coating of silicon nitride
4 _ electrode caps
5 = curr~nt conductor
6 = lasquer layer.
Embodiment-
_ .
A large number of cylindrical bodies
: ': ' ' ' :
of a ceramic material on the basis of aluminium o~ide
having a diameter of 205 mm and a leng-th o~ 5 mm are
exposed in a rotating drum to a gas current of 1 % by
:
volume of methane (CH4), remainder ~itrogen (N2) at1050C
- and a pressure of 1 atmosphere. After a carbon film of
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PHN 88l~6
30.12.1977
;7
approximately 0.08/um has been deposited the methane-
containing gas current is replaced by a gas current
consisting of 18% by volume of ammonia (NH3), 0,15% by
volume of silicon hydrid~ (SilI4) remainder ~ltrogen (N2)
and the temperature is raised to 900C. The bodies having
a silicon nitride film, 0,1/um thick, are provided with
metal electrode caps 4 which are pushed on to the ends,
so that the silicon nitride film 3 is locally removed
and a proper elec-tric contact with the carbon film is
obtained. Thereafter current conductors 5 are connec-ted
to the electrode caps by means of resistance welding.
Ther~after the resistors are brought
to the desired value by making a helical incision in the
carbon film whilst continuously measuring the resistance.
The finished resistors are thereaf-ter
coated with a single lacquer layer ~n the basis of an
epoxy resin.
Resistors obtained in this manner
having a resistance value of 1 M Q were immersed for 1
hour in water having a temperature of 100C. Thereafter
the resistors are charged in a room having a relative
humidity of 95% for one hour at the rated voltage (250 V)~
The increase in resistance was not more than 005%. With
resistors without silicon nitride layer bhe resistance
value increased by at least 10 %. However, some resistor
films appeared to have been interrupted. If the resistors
are ~harged in alr in su¢h circumstances bhat they become
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PHN 88l~6
30.12.1977
4~i7
red hot, the resistors according to the invention
appeared to return to the original value when cooled .
If no sllicon nitride layer is provided the carbon
film is fully burnt.
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